[qiqitori]

Easy in the case of ransomware or any other cases where a victim has to pay Bitcoins. The entity who pays the ransom/whatever sends the money to a Bitcoin address. This money is immediately marked as "confiscated" in cooperating jurisdictions. That money will be mixed in a tumbler (operating in a non-cooperating jurisdiction) with other Bitcoins, which get the same treatment. Or exchanged for a different cryptocurrency in an exchange that facilitates money laundering, and all cryptocurrency that is traceable that goes through that exchange is immediately marked as "confiscated".

You can keep using Bitcoins in non-cooperating jurisdictions, sure. But they'll automatically be less valuable or worthless because you can't use them in many countries.

People with development experience in Bitcoin could implement this in a jiffy. Doesn't even take a government really, you could probably build a startup whose main thing is to provide this information. Then people/exchanges/etc. could decide on their own if these Bitcoins have value or not.